Microwave Doppler transceivers are devices that transmit a Microwave pulse at a frequency in the GHz region of the electromagnetic spectrum, and receive return pulses that are reflect by objects. Stationary objects reflect a return pulse at a frequency equal to the transmitted frequency. On the other hand, an object that is in motion, towards or away, from the Microwave Doppler transceivers will shift the original frequency and reflect a return signal at a frequency that is offset by a particular frequency, based on the speed and direction of the object relative to the microwave Doppler source. This phenomenon is known as a Doppler shift.
Security systems utilize this Doppler shift to detect motion, which may indicate an unauthorized intrusion into the monitored area. However, Microwave Doppler transceivers are sensitive to fluorescent lights, which can cause false alarms and mask legitimate signals. Traditional filtering techniques using passbands in the range of 5 Hz to 500 Hz, are impractical because the noise falls within the passband frequency range. Anti-masking systems are equally sensitive to noise emanating from fluorescent lights, as well.
Fluorescent lights operate by supplying a high voltage pulse across a space filled with a gas that, once excited by the pulse, causes phosphor particles to fluoresce, thus emitting light. This process charges and discharges the gas, causing the gas particles to move back and forth. The Microwave Doppler transceiver readily detects the motion of the gas particles and interprets it as an intruder, resulting in a false alarm.
Solutions, such as hardware notch filters, are impractical for high volume low cost manufacturing and in addition, may remove too much of the desired signal. Presently, Microwave Doppler transceivers are designed to reject line noise by sampling at 50 Hz, creating a comb filter tuned to multiples of the sampling frequency.
In the U.S., and other regions of the world, the line frequency is set to 60 Hz, requiring a different sampling rate. Products designed for use in both 50 Hz countries and 60 Hz countries overcome this problem by including a DIP switch that the installer is required to set based on the local line frequency, thus allowing a single product to be sold in all regions. However, DIP switches are undesirable to customers, as they require time to set and introduce the potential for errors resulting from an incorrectly set DIP switch.
In some areas of the world frequency control of the 50 or 60 Hz line frequency may be imprecise. If the line frequency were not exactly 50 Hz, the 50 Hz sampling would introduce a low frequency alias that could be strong enough to produce a false signal. For example if the line were at 51 Hz, a 1 Hz alias would result that would not be completely attenuated from the 5 Hz analog high pass filter. A better solution would be to sample exactly at the line frequency, whatever that happened to be. In these cases, a DIP switch allowing selection of one of a predefined set of line frequencies is entirely inadequate